1. Field of the Invention
The present invention relates to a signal delay time measurement device which measures a signal delay time in a sound space in an audio system including a plurality of speakers.
2. Description of Related Art
For an audio system having a plurality of speakers to provide a high quality sound space, it is required to automatically create an appropriate sound space with much presence. In other words, it is required for the audio system to automatically correct sound field characteristics because it is quite difficult for a listener to appropriately adjust the phase characteristic, the frequency characteristic, the sound pressure level and the like of sound reproduced by a plurality of speakers by manually manipulating the audio system by himself to obtain appropriate sound space.
So far, as this kind of automatic sound field correcting system, there is known a system disclosed in Japanese Patent Application Laid-open under No. 2002-330499. In this system, for each signal transmission path corresponding to plural channels, a test signal outputted from a speaker is collected, and a frequency characteristic thereof is analyzed. Then, by setting coefficients of an equalizer provided on the signal transmission path, each signal transmission path is corrected to have a desired frequency characteristic.
In addition, a signal delay time of each signal transmission path corresponding to plural channels is measured, and a signal delay characteristic on each signal transmission path is adjusted. In a normal signal delay time measurement, when a processor in an automatic sound field correcting system outputs a measurement pulse, the processor simultaneously starts to receive a microphone input. A time period until a level of the microphone input first becomes larger than a predetermined threshold is determined as the signal delay time.
However, by a background noise in an environment to which a system is set, the background noise level may become larger than the threshold before an actual response of a measurement pulse reaches the processor, and it is problematic that the signal delay time shorter than the actual signal delay time is erroneously measured.
In that point, for preventing the error from occurring to the determination due to an effect of the background noise, there is proposed a technique for preventing an erroneous determination due to the background noise by measuring the level of the background noise in advance and setting the threshold to a level a little higher than the level of the background noise.
However, even if the technique is adopted, when a signal delay time of a certain channel is actually measured, the level of the background noise may become higher than the level of the background noise measured in advance. Therefore, the erroneous determination cannot often be prevented.